A power supply is a device or system that supplies electrical or other types of energy to an output load or group of loads. The term power supply can refer to a main power distribution system and other primary or secondary sources of energy. Power conversion refers to the conversion of one form of electrical power to another desired form and voltage, for example converting 115 or 230 volt alternating current (AC) supplied by a utility company to a regulated lower voltage direct current (DC) for electronic devices, referred to as AC-to-DC power conversion.
A switched-mode power supply, switching-mode power supply or SMPS, is a power supply that incorporates a switching regulator. While a linear regulator uses a transistor biased in its active region to specify an output voltage, an SMPS actively switches a transistor between full saturation and full cutoff at a high rate. The resulting rectangular waveform is then passed through a low-pass filter, typically an inductor and capacitor (LC) circuit, to achieve an approximated output voltage.
Conventional series-regulated linear power supplies maintain a constant voltage by varying their resistance to cope with input voltage changes or load current demand changes. The linear regulator tend to be inefficient. The switch mode power supply, however, uses a high frequency switch, the transistor, with varying duty cycle to maintain the output voltage. The output voltage variations caused by the switching are filtered out by the LC filter.
Linear power supplies and SMPSs can both be used to step-down a supply voltage. However, unlike a linear power supply, an SMPS can also provide a step-up function and an inverted output function. An SMPS converts an input voltage level to another level by storing the input energy temporarily and then releasing the energy to the output at a different voltage. The storage may be in either electromagnetic components, such as inductors and/or transformers, or electrostatic components, such as capacitors.
In general, an SMPS is classified as a rectifier, a voltage converter, a frequency converter, or an inverter, each according to the input and output waveforms. A rectifier has an AC input and a DC output. A frequency converter has an AC input and an AC output. An inverter has an DC input and an AC output. A voltage converter, also referred to as a current converter or DC-to-DC converter, has a DC input and a DC output.
Advantages of the SMPS over the linear power supply include smaller size, better power efficiency, and lower heat generation. Disadvantages include the fact that SMPSs are generally more complex than linear power supplies, generate high-frequency electrical noise that may need to be carefully suppressed, and have a characteristic ripple voltage at the switching frequency.
High-frequency ripple results when passing current through the transistor switches and then filtering the current with passive components. The frequency components of the ripple are dependent on both the switching frequency and the switching speeds of the semiconductor switches. The high-frequency ripple generates unwanted electromagnetic interference (EMI) and must be removed to a high degree for the converter to pass standard EMI requirements.
Conventional power converters pass EMI requirements by reducing the input and output ripple. Reduction is accomplished by the following methods: large filters, reduction of switching frequency, and/or reduction of switching speeds. Such techniques are commonly practiced in nearly all conventional power converters. However, use of each of these techniques comes with specific drawbacks. Use of large filters adds space and cost. Reduction of switching frequency increases the size of passive components and cost. Reduction of switching speeds reduces efficiency.
A variety of different DC-to-DC power converter configurations are currently in use, most of which are variations of a buck converter, a boost converter, and a buck-boost converter. Some versions of the buck converter include the push-pull converter, the forward converter, the half-bridge converter, and the full-bridge converter. A resonant power converter includes an LC circuit to shape the voltage across and the current passing through the transistor switches so that the transistor switches when either the voltage or the current is zero.
A configuration using a push-pull converter is similar to the half-bridge converter configuration except that the push-pull converter configuration center taps the primary transformer. A configuration using a full-bridge converter is similar to the half-bridge converter configuration except that the full-bridge converter includes two transistor switches coupled to each end of the transformer primary, as opposed to one end as in the half-bridge converter.
Another conventional power converter uses two interleaved hard-switched converter stages to reduce ripple. In an exemplary configuration, ripple is reduced by interleaving two boost converter power factor correction stages which are operating in critical conduction-mode. However, the interleaving of these converters only yields approximately a factor of 4 decrease in the ripple at the input of the converter. In hard-switched converters, such as critical-conduction-mode PFC (power factor correction) converters, the output is controlled via the duty cycle, not the frequency. Changes to the switching frequency have no effect on the output. It is therefore relatively easy to interleave two hard-switched converters that have slightly different component values. On the other hand, due to the tolerance of components in two resonant converters, it is extremely difficult, if not improbable, to match the resonant frequencies of those two converters making it problematic to operate the two resonant converters at a frequency equal to the resonant frequency of each one.
U.S. Pat. No. 4,695,933 is directed to a multi-phase purely sinusoidal resonant converter such that the outputs are summed. There is no mention of how to make sure all converters have the same resonant frequency. U.S. Pat. No. 6,583,999 describes a boost pre-regulator with series resonant half-bridge and also describes a boost pre-regulator followed by two-phase series resonant converter. U.S. Pat. No. 6,970,366 describes a generic multi-phase resonant converter using sinusoidal waveforms. There is no mention of how to match resonant frequencies of each section. Each of these three patents share the same deficiency. The resonant frequency of the converter is determined by values of the inductors and capacitors that form the resonant tank. In practice, values of inductance and capacitance vary around a nominal value with a tolerance, typically on the order of 5% or 10%. The resonant frequency of each converter will therefore vary. For example, in the typical case, the resonant frequency of a converter is equal to 1/(2π√{square root over (LC)}) so that a +5% variance in both capacitance and inductance will lead to a −5% variance in the value of resonant frequency. In each of the three patents cited above, a converter designed to operate at a predetermined switching frequency cannot be guaranteed to operate at resonance unless one or both resonant tank components (the inductor and the capacitor) are hand-selected for each unit, or unless the switching frequency is adjusted for each unit.
U.S. Pat. No. 6,487,095 is directed to a multi-phase resonant converter having a variable resonant tank that uses a tunable inductor, and a synchronous rectifier coupled to the output. In order for an inductor to be tunable, it must operate at relatively high flux densities. This is not an efficient area of operation for an inductor, so the ability to tune the inductance comes at the expense of converter efficiency.